In recent years, the use of ink-jet system to generate images has rapidly growth. The improvement in both printers and ink-jet recording media performances allows to obtain images similar to conventional silver halide photographic ones. At the same time, due to spreading speed increase in printing systems, there is the need to use recording media able to absorb all ink volumes in very short time. The ink high speed absorptivity can be achieved using an ink-jet recording material comprising a porous layer which is able to rapidly remove the fresh spread ink from the surface of the material, while the absorption capacity can be adjusted varying the thickness of the receiving layer.
Even if the ink-jet record materials having porous layers present a better ink absorptivity than ink-jet record materials utilizing swelling system layers, the images printed on porous system may suffer of oxidation deterioration of coloring matter component under natural air exposure. The gas circulation in porous materials supplies a continuous flux of oxidizing compounds, such as nitrogen oxides, sulfur oxides, oxygen or ozone gas, that are directly in contact with the coloring matter in the porous layer. Accordingly, images obtained by ink-jet recording materials have inferior properties in terms of storage characteristics, such as light resistance, water resistance and gas resistance, compared to images obtained by silver halide photographic materials.
The use of conventional antioxidant compounds provides a limited protection in the time that is vanished when the compound is consumed. Most of natural or synthetic antioxidants, like vitamin C, vitamin E, substituted hydroquinone, exhibit positive activity to prevent image discoloration due to oxidation, but have the disadvantage of generating colored substances, as reaction products, that visibly modify the printed image.
Various attempts have been done in the art to solve this color fading problem; in some cases, a color fading inhibitor compound has been added to the ink composition (hereinafter antifading compound). European Patent 875,544, for example, discloses an ink composition comprising a colorant, water, a water-soluble organic solvent, and a fine particle of a polymer, the polymer constituting the fine particle having a film-forming property and, at the same time, an ultraviolet absorbing capacity and/or a light stabilizing capability; Japanese Patent application 11-315,234 discloses an ink composition containing a triazine compound and a sterically hindered amine compound; Japanese Patent application 05-239,389 discloses a recording liquid comprising a dye, pure water and a light stabilizer capable of obtaining high-grade stabilized images.
As other kind of solution, a transferring protective covering material comprising a fluorescent whitening agent, an ultraviolet light absorber, and a light stabilizer to cover a printed image is disclosed in U.S. Pat. Nos. 4,756,963; 5,954,906 discloses a transferring protective covering material comprising a flexible substrate, an adhesive layer containing a light stabilizer and a solid resin layer. EP 1,186,439 discloses a surface treatment method for recorded matter, such as images recorded on a recording medium and the recording medium that contains such images, which can improve the weather characteristics of the images, such as light resistance and gas resistance. This method is characterized by subjecting the recorded matter to an after-treatment, such as spray treatment, coating treatment, immersion treatment, to form an overcoat layer applied onto the surface of the recorded matter. This overcoat layer contains an aqueous solution containing a water-soluble resin, a light resistance improving agent and an ink fixing agent; the overcoat layer protects the surface of the recorded matter preventing the gas present in the air to penetrate into the recorded matter, and thus improving the color fading due to light and gas presence. On the other hand, a re-wet liquid solution applied to a dried porous basecoat is disclosed, for example, in U.S. Pat. Nos. 6,475,612, 6,432,523 and 6,423,375, wherein a process that allows the production of multi-layer ink-receiving materials in which one or more topcoats can be applied to a porous basecoat comprising a plurality of pores to produce a uniform and defect-free coating layer is disclosed. However, both the after-treatment solution and the re-wetting solution described above involve the use of a treatment agent; this means that the treatment agent must be incorporated into an additional composition to form the protective layer to be sprayed or coated onto the surface of the recorded matter, before or after that he image is obtained, with the consequent problems related to choose the proper elements contained in the additional composition, how to introduce and mix them, how to store and treat the final composition, and so on.
Hence, it would be preferable to add the antifading compounds directly into the recording elements, without the need to prepare any additional compositions to be sprayed or coated onto the recorded matters. Many patents in the art disclose recording elements wherein ink-receiving layers contain chemical compounds to prevent color discoloration. EP 1,120,281 discloses an ink-jet recording material comprising pigment particles dispersed in a binder with average particle size of 1 micron or less. The ink receiving layer comprises light resistant enhancing agents for image selected from phenolic compounds, boric acid, borate salts and cyclodextrin compounds. EP 1,008,457 and U.S. Pat. No. 6,391,428 disclose an ink-jet recording sheet having an image preserving layer comprising anionic colloidal silica and zinc oxide particles; the ink receiving layer also comprises specific sterically hindered amines compounds having alkoxy groups. For example, WO 2002-055,618 discloses the use of specific water soluble sterically hindered amine compounds which contain an oxyl or hydroxyl group on the nitrogen atom; EP 1,031,431 discloses a recording medium comprising a specific sterically hindered amine compound having an alkyl group bonded to nitrogen atom of a piperidine ring with interposition of oxygen; EP 1,134,087 discloses a recording medium comprising an ink receiving layer containing specific sterically hindered amine compounds exhibiting solubility in water of 0.01 to 5%; Japanese Patent applications 61-146,591; 11-245,504 and 2000-247,015 describe recording sheets containing specific sterically hindered amine compounds which show improved light fastness and water resistance.
However, inkjet receiving materials containing antifading compounds incorporated in the same layers containing filler, such as alumina hydrate or silica compounds, cannot be obtained due to the interactions between the antifading compounds and the filler compounds that cause incompatibilities during coating process and unacceptable coating defects.
In spite of the number of tentative solutions in the art, there is still the need to have an ink-jet recording material which improves weather storage characteristics over time, such as air resistance, preventing oxidation deterioration of coloring matter component of the printed image during long term exposure to natural air, without causing coating problems and coating defects.